Start working on strategy evaluation

marketsim/marketsim.py

@@ -75,6 +75,7 @@ def get_portfolio_value(holding, prices):
 def handle_order(date, order, holding, prices, commission, impact):
     """Process the order."""
     symbol, order, shares = order
+    assert(shares > 0) # Can only buy or sell positive amount of shares.
     adj_closing_price = prices[symbol]
     cost = shares * adj_closing_price
     # Charge commission and deduct impact penalty